a. Field of the Invention
The invention relates to a resistivity measuring instrument and more particularly to a compact eddy current resistivity measuring instrument with gauge heads equipped with sensors for conductance and thickness measurements of test articles of unknown conductance and thickness.
b. Prior Art
In a co-pending patent application, Ser. No. 903,493, an apparatus is disclosed for measuring distance between a gauge head orifice and the front planar surface of an object by sensing acoustic pressure near the gauge head orifice. The acoustic pressure is initially generated by an audio oscillator activating a speaker which emits acoustic waves through a front orifice. Acoustic pressure is sensed through a second orifice, proximate to the first, and the pressure is converted to an eletrical signal which is processed to yield a distance signal. Two similar distance gauges spaced apart on opposite sides of a planar member at a known distance are used to compute the thickness of the planar member. While thickness measurements have application in many fields, one particular field of interest is in the measurement of resistivity of sheet materials in which eddy currents may be induced.
In an article entitled "Contactless Measurement of Semiconductor Conductivity by Radio Frequency-Free-Carrier Power Absorption" by G. L. Miller, D. A. H. Robinson and J. D. Wiley of Bell Laboratories, reported in Review of Scientific Instruments, vol. 47, No. 7, July, 1976, p. 799-805, a conductance gauge is disclosed in which a pair of ferrite cores are spaced on opposite sides of a sheet material, a semiconductor slice, for inducing eddy currents therein so that conductance can be determined. The ferrite cores are solenoids which are axially aligned and which cooperate with each other to form an axial magnetic field. The two solenoids are mutually connected through a capacitor forming a tank circuit which is resonant at a radio frequency. An oscillator is energized by a d.c. current which maintains a constant voltage across the resonant circuit. To measure conductance the d.c. current necessary to maintain the axial magnetic field is first read without any magnetically susceptible material in the magnetic field. Then the current is read with a test article in the field. The test article in the field damps the resonance of the circuit, causing additional Q loading. The authors of the above mentioned article have found that the additional power absorbed by the sheet material is accurately proportional to the material conductance.
The gauge described by Miller, et al. measures only conductance and in many applications it is important to know resistivity instead. Resistivity may be obtained by dividing thickness by the conductance of the test article. In the prior art, thickness was measured by air gauges or by other means which require a separate measurement, using a separate instrument.
One of the problems recognized by Miller, et al. is that at high radio frequencies at which conductivity measurements are made, there is a certain amount of electrostatic coupling to the sheet material. Miller, et al. recommend cementing a disc of conductive paper across the ferrite core as a solution to the problem of electrostatic coupling or capacitance which tends to change the character of the tank circuit by additional Q loading. The problem with conductive paper is that if it is too conductive the Q of the tank circuit significantly drops. If the paper is not conductive enough, it does not shield against electrostatic loading.